WEST MICHIGAN — A sticky substance clinging to cars, plants, patio furniture and virtually anything left outside this summer is nothing to worry about, one expert says, despite residents’ fears it might be pollution raining from the sky.

What’s causing the phenomenon has six legs and lives in trees — it’s aphids, which for some reason may be more numerous this summer, said Shane Van Oosterhout, master gardener program coordinator for Muskegon and Ottawa counties’ Michigan State University Extension offices.

He said aphids and similar insects suck sap from plants, then excrete a sugary substance known as “honeydew.” Mold spores floating in the air can settle and grow in the aphid droppings, sometimes giving the gunk a dark color suggestive of tar.

Van Oosterhout said the gunk forms naturally every year, but because of the amount produced this summer some residents believe it is a toxic pollutant.

“I’ve received some pretty hilarious conspiracy theories,” he said, adding that the substance is not harmful to humans, animals, or plants.

Van Oosterhout said the increased honeydew comes from a high aphid population this year, possibly due to the weather or reduced populations of aphid-eating insects.

The gunk has sent some Muskegon and Ottawa county residents scrambling for answers, calling their city governments or even the Environmental Protection Agency.

Roosevelt Park resident Amanda Farber said she called the EPA because she was concerned that the gunk was coming from a nearby factory.

“I know neighboring areas are not getting this film/residue,” she said. “It reminds me of hairspray. ... I find it hard to believe that it is aphid droppings.”

Muskegon resident Becky Ray said she and her husband first noticed residue on their cars in early July.

“At first, we thought maybe someone had sprayed our cars with soda pop,” she said. “Then we noticed that it keeps coming back after a car wash.”

Some have reported that the film on their cars is dark; Others say it’s not. Roosevelt Park resident Joel Coston said the sticky substance on his car is clear.

“At times, it is so thick you can barely see out of your windows when you get into your car in the morning,” he said.

Roosevelt Park City Manager David Boehm said he has been getting calls from across the city for the past two or three weeks. He said he is referring residents to the MSU Cooperative Extension.

“We’re not tree experts. We’re not insects experts,” he said.

The Grand Haven city manager’s office had received one call about the gunk as of Wednesday afternoon, and other area cities reported no calls from concerned residents.

Van Oosterhout said he didn’t know if the goo also is appearing outside West Michigan.

Sarah Taylor, of Grand Haven, said she and her sister-in-law, who lives in Saginaw, have both found the clear gunk on their windows, and that prompt washing keeps it from turning black because mold doesn’t have time to grow.